Whos from england and where abouts

In the diagram below, ab and bc are tangent to o. what is the measure of

Can someone me with this plz? ! i'm putting 20 points on it!

All of the following are reasons why deserts extremely susceptible to wind erosion except a. they lack major root systems b. they contain thin, dry soil c. they have very few plants and trees d. they are extremely hot and lack water